1. Physics Department, University of California, Berkeley, California 94720-7300, USA
2. Space Sciences Laboratory, University of California, Berkeley, California 94720-7450, USA

Correspondence to: Linghua Wang^1,2 Correspondence and requests for materials should be addressed to L.W. (Email: windsound@ssl.berkeley.edu).

Abstract

The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun¹. Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma² contains only 20 per cent of the energy released by the termination shock, whereas energetic particles³ above 28 keV contain only 10 per cent; 70 per cent of the energy is unaccounted for, leading to speculation^{2, 3} that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic (4–20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source 60° wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at 11 keV instead of 4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing 70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.

1. Physics Department, University of California, Berkeley, California 94720-7300, USA
2. Space Sciences Laboratory, University of California, Berkeley, California 94720-7450, USA

Correspondence to: Linghua Wang^1,2 Correspondence and requests for materials should be addressed to L.W. (Email: windsound@ssl.berkeley.edu).

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